A type of system known to the Applicant for filtering particles from a liquid comprises:
(a) a filter comprising: an enclosure
a filter chamber within said enclosure having an inlet and an outlet for said liquid, a portion of the wall of said chamber being constituted by a porous member of sintered material having an inside surface facing the inside volume of the chamber and an outside surface facing the other way, the inside and outside surfaces being parallel to each other, the porous member including a membrane one of whose surfaces constitutes said inside surface, the pores in the membrane being smaller than the particles to be filtered from said said liquid, each pore being in communication with adjacent pores, and said enclosure surrounding said outside surface, said enclosure being provided with an outlet opening;
(b) means for inserting the liquid into the chamber via its inlet and at a first constant pressure;
(c) means for impressing a predetermined speed parallel to the said inside surface to the liquid contained inside the chamber, a portion of the liquid passing through the porous member and being received in the enclosure;
(d) a pump connected to the outlet from the chamber and to a first tank for removing another portion of the liquid that has circulated inside the chamber;
(e) liquid flow rate measuring apparatus series connected between the opening of the outlet from the chamber and a second tank;
(f) a servo-control circuit receiving information concerning the measured flow rate and capable of controlling the pump to maintain a constant predetermined ratio between the measured flow rate and the pump flow rate; and
(g) a controllable liquid backpressure unit connected between the opening of the enclosure outlet and the measuring apparatus for applying, during a predetermined period of time, a second constant liquid pressure on the outside surface of the porous member, in opposition to the first constant pressure.
In this system, the porous member may, for example, be constituted by a ceramic tube including a microporous tubular membrane, and the means for impressing a predetermined speed parallel to the inside surface of the tube may comprise a cylindrical core rotatably mounted inside the tube. The back pressure exerted by the liquid backpressure unit having the effect of backwashing the tube by removing from the membrane those particles which have become pressed against the said inside surface during filtering.
However, such a system has a drawback.
When the liquid backpressure unit is put into operation after a relatively long period of filtering, it may be that the membrane is no longer capable of being backwashed by the back pressure from said unit. It is then necessary to disassemble the porous member from the filter for regenerating the membrane, e.g. by pyrolysis in a high temperature oven or by chemical washing; in either case the filtering operation must be interrupted for some time. To remedy this drawback, an arbitrary and fairly high rate of backwashings may be instigated. However, each time the liquid backpressure unit is used, a certain quantity of filtered liquid is returned upstream from the filter, thereby increasing filter time and reducing overall system efficiency in terms of volume of liquid filtered per unit time.
Preferred embodiments of the present invention mitigate this drawback.